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The exact cause of Crohn's disease is unknown. Most researchers think it's caused by a combination of factors.
These are thought to be:
There's no evidence to suggest a particular diet can cause Crohn's disease, although dietary changes can control certain symptoms and may be recommended by your specialist or dietitian.
Read about treatment for Crohn's disease for more information.
There's evidence to suggest genetics plays a role in the development of Crohn's disease.
Researchers have identified more than 200 different genes that are more common in people with Crohn's disease than in the general population.
There's also evidence that Crohn's disease can run in families. About 3 in 20 people with the condition have a close relative (mother, father, sister or brother) who also has Crohn's disease. For example, if you have an identical twin with the condition, you have a 70% chance of developing it.
The fact that Crohn's disease is more common in some ethnic groups than in others also suggests that genetics plays an important role.
The immune system provides protection against harmful bacteria that could potentially find their way into the digestive system.
The digestive system is also home to many different types of so-called "friendly bacteria" that help to digest food. The immune system usually recognises these bacteria and lets them do their job without attacking them.
However, in Crohn's disease, it seems that something disrupts the immune system, which sends a special protein known as tumour necrosis factor-alpha (TNF-alpha) to kill all bacteria, regardless of whether they're friendly or not. This causes most of the inflammation associated with Crohn's disease.
In certain genetically susceptible individuals, a previous childhood infection may lead to an abnormal immune response, causing the symptoms of Crohn's disease.
Aside from family history and ethnic background, smoking is the most important risk factor for Crohn's disease. Smokers are twice as likely to develop the condition than non-smokers.
Furthermore, people with Crohn's disease who smoke usually experience more severe symptoms and are much more likely to require surgery.
Read about how to get help to stop smoking.
There are two unusual aspects of Crohn's disease that have led many researchers to believe that environmental factors may play a role. These are explained below:
This suggests there is something associated with modern Western lifestyles that increases a person's risk of developing the condition.
One theory to explain this is known as the hygiene hypothesis. It suggests that as children grow up in increasingly germ-free environments, their immune system doesn't fully develop because of a lack of exposure to childhood infections. However, there's little in the way of hard scientific evidence to support this theory.
An alternative theory is the cold-chain hypothesis, which suggests that the increase in Crohn's disease cases might be linked to the increased use of refrigerators after the Second World War.